The present invention relates to a coupling assembly and in particular, although not exclusively, to a coupling assembly for use in connecting fluid passages in oil and gas pipe lines.
When extracting oil and gas from well heads beneath the sea, it is necessary to couple two fluid passages together. WO 2007/017677 discloses a particularly suitable coupling assembly and the contents thereof are hereby incorporated by reference. The known coupling comprises a male member and female member. The male member includes a probe that is inserted into a socket of the female member. The probe and socket are generally circular in cross-section and the two parts can be mated by relative movement of the two parts along a first longitudinal direction. Both the male and female parts include through bores. When the coupling is mated the through bores are aligned to provide a fluid conduit through the coupling. Fluid is maintained within the fluid conduit due to sealing rings that are provided either side of the intersection of the two through bores and between the socket and probe.
In WO 2007/017677, unintentional de-coupling of the coupling assembly is prevented using a shear pin that is inserted through a hole on either side of the socket and an aligned hole in the probe. The pin extends at right angles to the first longitudinal axis. When a force is exerted to urge the male and female members apart, the shear pin resists the separation. In order to allow the pin to be inserted, the holes are necessarily larger than the pin. Thus some movement is observed as the male member and female member pull apart and relax. When the force urging the male and female members apart exceeds a predetermined force, the shear pin is designed to fracture on either side of the probe and allow decoupling. This force is determined as the break out strength of the coupling and is designed to avoid damaging equipment or pipelines. Other break out mechanisms are also disclosed operating a similar principle of requiring a shear fracture to occur on each side of the probe.
Whilst the coupling assembly disclosed in WO 2007/017677 provides a suitable releasable coupling assembly, it has been found that during break out of the coupling assembly false activations may occur, the probe may jam within the socket when withdrawing and the setting of the breakout strength may be imprecise. For instance, if more than one shear component is provided, it is natural for one to shear at a lower force than the other. This causes the weakest component to shear first which transfers the load to a neighbouring component. Thus the components tend to fail in a “zip like manner”. This so called false activation makes it hard to design the coupling assembly to breakout at a precise force. Furthermore, because a part of the probe is released before another part, the withdrawal of the probe from the socket may be caused to become angled to the longitudinal axis which can cause the probe to jam within the socket.